This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-232066, filed Aug. 19, 1999; and No. 2000-239481, filed Aug. 8, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a color cathode ray tube, and more particularly, to a color cathode ray tube in which the outer surface of an effective portion of a panel is flattened.
In general, a color cathode ray tube comprises a vacuum envelope that includes a panel, which has a substantially rectangular effective portion, and a funnel bonded to the panel. Formed on the inner surface of the effective portion of the panel is a phosphor screen having three-color phosphor layers, which glow red, blue, and green, individually. A shadow mask having a large number of electron-beam passage apertures is opposed to the phosphor screen. Located in a neck of the funnel, moreover, is an electron gun for emitting three electron beams toward the screen.
In the color cathode ray tube constructed in this manner, the three electron beams emitted from the electron gun are deflected by means of magnetic fields generated by a deflector, which is mounted on the outside of the funnel, and scan the phosphor screen horizontally and vertically through the shadow mask, whereupon a color image is displayed on the screen.
For improved visibility, according to the modern color cathode ray tube of this type, the outer surface the effective portion of the panel tends to be flattened. As the outer surface of the effective portion is flattened in this manner, it is necessary also to flatten the inner surface of the effective portion, and besides, the effective surface of the shadow mask that is opposed to the phosphor screen on the inner surface of the effective portion.
If the conventional shadow mask, having the spherical effective surface, is flattened in a manner such that its curvature radius is simply increased, however, the so-called doming occurs when high-density electron beams are emitted from the electron gun to display a high-brightness image. The doming is a phenomenon that the shadow mask undergoes substantial local thermal expansion and bulges toward the phosphor screen when it is hit by the electron beams. The doming of the shadow mask causes lowering of color purity.
If the shadow mask is flattened, moreover, its strength for curvature retention lowers, so that the mask may be deformed by impact that acts thereon during the manufacture or transportation of the color cathode ray tube. In consequence, the cathode ray tube may possibly be rendered defective.
As means for easing the above problems, a panel is described in Jpn. Pat. Appln. KOKAI Publication No. 9-245685. This panel is designed so that the outer surface of its effective portion is flat, the major-axis-direction curvature radius of the inner surface is substantially infinite, and the minor-axis-direction curvature radius is substantially fixed. Proposed in Jpn. Pat. Appln. KOKAI Publication No. 10-199436, moreover, is a shadow mask of which the effective surface is shaped corresponding to a flattened panel.
The panel and the shadow mask described above ensure certain effects to cope with the aforementioned problems. To improve the visibility of the color cathode ray tube further, however, the inner surface of the effective portion of the panel and the effective surface of the shadow mask should be flattened additionally. The flattened shadow mask requires higher strength for curvature retention and a securer measure to counter doming.
The present invention has been contrived in consideration of these circumstances, and its object is to provide a color cathode ray tube with improved visibility, in which the strength of a shadow mask for curvature retention is improved and doming is reduced.
In order to achieve the above object, a color cathode ray tube according to the present invention comprises: an envelope including a panel with a substantially rectangular effective portion having a substantially flat outer surface and a funnel bonded to the panel; a phosphor screen on the inner surface of the effective portion; a shadow mask having a substantially rectangular effective surface opposed to the phosphor screen and formed with a large number of electron-beam passage apertures, the effective surface being a curved surface projecting toward the phosphor screen; and an electron gun located in a neck of the funnel, for emitting electron beams toward the phosphor screen, the shadow mask and the panel sharing a center through which a tube axis passes and major and minor axes passing through the center and extending at right angles to the tube axis and to each other, the shadow mask being formed so that there are relations:
1 greater than RVmin/RVmaxxe2x89xa70.3,
where RVmin and RVmax are minimum and maximum values, respectively, of the minor-axis-direction curvature radius of the effective surface on the minor axis, and
1 less than RHC/RHAxe2x89xa63,
where H is the distance between the opposite ends of the effective surface in the direction of the major axis, RHA is the average major-axis-direction curvature radius of the effective surface on the major axis, defined by H/2 and a depression zH of each major-axis end of the effective surface in the direction of the tube axis, compared to the center of the effective surface, and RHC is the major-axis-direction curvature radius in the center of the effective surface.
According to the color cathode ray tube of the invention, the minor-axis-direction curvature radius of the effective surface of the shadow mask on the minor axis gradually decreases from the center of the effective surface toward the minor axis end.
According to the color cathode ray tube of the invention, moreover, the major-axis-direction curvature radius of the effective surface of the shadow mask on the major axis gradually decreases from the center of the effective surface toward the major axis end.
According to the color cathode ray tube constructed in this manner, the strength of the shadow mask for curvature retention can be improved, and doming can be reduced. Further, the depression of the peripheral portion of the effective surface of the shadow mask in the direction of the tube axis is reduced, and therefore, the depression of the peripheral portion of the inner surface of the panel in the tube axis direction is reduced. Thus, the outer surface of the effective portion of the panel is flattened, and the difference in wall thickness between the central portion and the peripheral edge portion of the effective portion is reduced, so that the visibility can be improved.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.